The present invention relates to a pump unit, and particularly relates to a variable flow pump unit. Specifically, the present invention relates to a pump unit where the flow to a device or system supplied by the pump can be controlled by adjustment of a variable orifice in the outlet of the pump.
It is known that flow from a pump to a fluid device or system can be controlled by providing a variable orifice in the system between the pump and the device and controlling the flow from the pump by sensing the pressure drop across that orifice and effecting bypass of fluid from the pump outlet to the pump inlet in accordance with the pressure drop. Thus, by adjusting the orifice, pump output is changed and flow to the device or system is varied.
The present invention is directed to a compact, practical pump unit including a variable orifice, bypass valve and pump. Further, and more specifically, in accordance with the present invention, by adjustment of the variable orifice, the amount of fluid which is bypassed from the outlet of the pump to the inlet of the pump by the bypass valve is controlled and thereby the amount of fluid that flows to the system is controlled. Further, since the bypass valve responds to pressures acting thereon from opposite sides of the orifice, the bypass valve will shift in order to attempt to maintain a fixed output from the pump to the system being supplied with fluid by the pump.
Further, in accordance with the present invention, the bypass valve functions to relieve excessive pressures in the nature of a pressure relief valve. The bypass valve comprises a spool valve which has one end face which is acted upon by the pressure immediately downstream of the pumping elements (upstream of the variable orifice). The other end of the spool valve is exposed to pressure in a pilot pressure chamber which is communicated with pressure on the downstream side of the variable orifice. Accordingly, the spool valve moves in response to pressure changes on opposite sides of the variable orifice in an effort to maintain the fixed output from the pump.
In addition, the spool valve carries a pilot valve which communicates with the pilot pressure chamber. When the pressure in the pilot pressure chamber exceeds a predetermined value, the pilot valve moves to an open position, and communicates the pilot pressure chamber to the pump inlet. As a result, the pressure in this pilot pressure chamber is reduced. Further, the passage leading from downstream of the variable orifice to the pilot pressure chamber is constructed such that it restricts the rate of pressure buildup in the pilot pressure chamber when the pilot pressure chamber is vented by opening of the pilot valve. As a result, the pressure in the pilot pressure chamber is significantly reduced when the pilot valve opens and the spool will move to a pressure relief position by the force of the outlet pressure acting thereagainst. As a result, a significant amount of fluid will be bypassed from the outlet to the inlet for purposes of pressure relief. Once the conditions which created the excessive pressure are overcome, the spool valve will immediately return to its flow control position for purposes of maintaining the fixed output from the pump unit.